fix(firestore): preserve update timestamps in list_sessions

[kkj333]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):
Closes: #5632

Problem

FirestoreSessionService.list_sessions() always sets Session.last_update_time to 0.0, even when Firestore documents contain updateTime.

Solution

Use the same timestamp conversion logic as get_session() in list_sessions():

• if updateTime is a datetime, use .timestamp()
• otherwise, attempt float(updateTime)
• fallback to 0.0 when conversion fails
  This keeps list responses consistent with get_session() and preserves accurate update timestamps.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.
  Updated tests:
• test_list_sessions_with_user_id
• test_list_sessions_without_user_id
  Local pytest summary:
• tests/unittests/integrations/firestore/test_firestore_session_service.py
• 17 passed
  Manual End-to-End (E2E) Tests:
  Reproduced with a mocked Firestore client where session docs include updateTime; before fix last_update_time was 0.0, after fix it matches the Firestore value.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

[google-cla]

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[kkj333]

The failing check is caused by the triaging workflow configuration, not by this PR changes.

adk_pr_triaging_agent fails with:
404 NOT_FOUND: models/gemini-2.5-pro is not found for API version v1beta

Could a maintainer please update the triaging agent model (or rerun after fixing the workflow config)?

[rohityan]

Hi @kkj333 , Thank you for your contribution! We appreciate you taking the time to submit this pull request. Your PR has been received by the team and is currently under review. We will provide feedback as soon as we have an update to share.

[rohityan]

Hi @Jacksunwei , can you please review this.